Young magician Gywn and his friend Nia have been warned to stay away from Emlyn Llewelyn, the strange boy who claims his mother lives on the moon. And yet, a mysterious magic continues to draw them to him. But why? It's up to Gwyn and Nia to solve the mystery, with the help of Arianwen, the Snow Spider.

Gwyn can feel danger coming in the wind. Somehow he knows the warnings have to do with the broken toy horse that holds the evil spirit of a prince who lived long ago. When Gwyn discovers that the prince's dark soul has escaped from the horse and is seeking revenge Gwyn, Emlyn, and Nia have to figure out how to save the mysterious soldier who claims to be Nia's distant cousin. With the help of the Snow Spider, can they recapture the prince's soul without hurting the Chestnut soldier?

Gwyn can feel danger coming in the wind. Somehow he knows the warnings have to do with the broken toy horse that holds the evil spirit of a prince who lived long ago. When Gwyn discovers that the prince's dark soul has escaped from the horse and is seeking revenge Gwyn, Emlyn, and Nia have to figure out how to save the mysterious soldier who claims to be Nia's distant cousin. With the help of the Snow Spider, can they recapture the prince's soul without hurting the Chestnut soldier?

On his ninth birthday, young Gwyn (Osian Roberts), who lives on a remote hill farm in Wales, receives five strange gifts from his grandmother (Siân Phillips): a piece of seaweed, a yellow scarf, a tin whistle, a metal brooch and a small broken horse. Gwyn offers the brooch to the wind and receives back a tiny silvery spider - Arianwen, the snow spider - confirming that, as his grandmother had already guessed, he has inherited magical powers from his Celtic ancestor Gwydyon, a powerful magician whose exploits are described in the fourth book ("Math Son of Mathonwy") of the Mabinogion. With the help of the snow spider, Gwyn embarks on adventures involving other worlds of snow and silver, as he attempts to solve the five-year-old mystery of his sister Beth's disappearance in a snow storm.

Shada: a prison built by the Time Lords for defeated would-be conquerors of the universe. A scientist named Skagra needs the help of one of the prison's inmates. He finds nobody knows where Shada is anymore, except one aged Time Lord who has retired to Earth, where he is a professor at St Cedd's College. Luckily for the universe, Skagra's attempt to force the information out of Professor Chronotis coincides with a visit by the professor's old friend, The Doctor.

The Doctor arrives on a planet where two tribes, the savage Sevateem and the technically brilliant Tesh, are at war. He meets Leela, an exile from the Sevateem, and discovers that the tribes' god of evil is apparently himself...

The Doctor and Peri arrive on Karfel, a planet ruled by fear where the enemies of its ruler, the despotic Borad, are banished into a time tunnel known as the Timelash.

Far in the distant future, Earth has become uninhabitable, forcing Mankind to colonise first Mars and then Pluto. No longer the coldest planet in the solar system, Pluto is now warmed by artificial suns. The Doctor, Leela and K9 arrive to discover the exploitation of the Megropolis people by the ruling elite, lead by the Collector. Deep in the Undercity, a small group of revolutionaries plot to overthrow the company and the Doctor is forced to fight the oppression of the people using Fire against Fire...

The Doctor and Romana learn the second segment of the Key to Time is on the planet Calufrax. Yet they arrive on a planet called Zanak, which has been hollowed out and fitted with hyperspace engines, allowing its insane, half-robot Captain to materialise it around smaller planets and plunder their resources.

The Fifth Doctor, Tegan and Turlough arrive on Sea Base 4, a nuclear warhead station under the sea that has some very nasty neighbours.